The findings of a new study suggest two ways to effectively address the problem that birth control pills may not work as well in obese women, compared to women of a normal body mass index.
Birth control pills are a one-size-fits-all method, researchers say, but as the population has increased in weight, concern has grown about how well the pill works for obese women. Studies have consistently found that obesity has a negative impact on drug levels in the body, which may in turn affect how well the pill prevents pregnancy.
"Birth control pills have been shown in a large population study to fail at a higher rate in women who are obese," said Ganesh Cherala, an assistant professor in the Oregon State University/Oregon Health & Science University College of Pharmacy.
"Our original studies were focused on why this might occur," Cherala said, "and we found that obesity changes how a woman's body clears contraceptive hormones."
It takes longer for the pill to reach a steady state level in obese women, with possible impacts on efficacy of the birth control, and putting them at greater risk for a pill failure if they forget to take a pill or take it later.
In order to offset these changes, Cherala and Dr. Alison Edelman, an associate professor of obstetrics and gynecology at Oregon Health & Science University, studied two alternative strategies. They found that either a slight increase in the pill dose, from a very low dose to a low dose pill; or using the pill continuously without a "period week" off, appeared to counteract the changes that obesity causes.
This, in turn, may provide improved pregnancy prevention for women of differing weights who use the pill, the researchers said. Their work is published in Contraception, a professional journal, and was supported by the National Institutes of Health.
"Since oral contraception remains one of the most popular forms of birth control in the United States and the majority of our population is obese or overweight, it's important to find methods of contraception that work for all women, no matter what their weight," Edelman said.
"The strategies that we studied can be, and are currently being used by women, but now we know that they help to counteract the adverse effects of weight on contraceptive hormones," she said.
For obese women, simply shifting to an alternative form of birth control is an option, the researchers said. But they also pointed out that oral contraceptives are the most preferred form of birth control and that a woman's individual preference influences her adherence and continuation with any method.